The calibur charge attack is great because it combos well with the shivermist (delivering a ton of damage to a single or a cluster of targets). The idea isn't necessarily to use it like you would use a bomb (although in practice that's what it feels like), rather, to use it to shore up your weaknesses. The only thing that is "difficult" to clear using ONLY bombs are gun puppies because bombs are very inefficient and can result in aggroing more than you want. It's not really difficult as much as it is inefficient though. The real problem with clearing gun puppies with only bombs is aggroing other puppies or dealing with multiple other enemies while you slowly chip away at the puppy. Because of this, the calibur is a great compliment to bombers since the charge attack can essentially one-shot puppies all the way to the core. Blast bomb charge might be a lot faster but you're going to need 4+ bombs to deliver the same payload to a single target. Really though, you'll want anything that can take care of puppies easily and there are a ton of swords or guns that can do the trick.
>because bombs just don't do the job very well
I disagree with you here. I find that lumbers are easily dealt with haze bombs. Their inability to turn fast enough means you can keep stacking haze bombs on their weak side without leaving yourself vulnerable. Kats move a lot but haze bombs, sun shards, dark briar, and nitronome are all viable ways to deal with them.
Sealed sword is fine, it upgrades to the divine avenger which gives you a straight line attack mechanism that bombs and swords generally lack.
One of my biggest problems as a bomber are the less mobile monsters. You'd think the more mobile ones would be more of a problem (and I guess greavers qualify, cause they suck), but when stuff moves around, you can get it into blast range. Things that can't or don't want to move means you have to go to them, giving them the chance to whump you.
Kats, Puppies, and Lumbers. I keep an alchemer around for those (thankfully, all are weak to elemental), because bombs just don't do the job very well. The charge attack on the (Divine) Avenger will give you the long distance killing power (and also thankfully, the DA hits with elemental) for things like that.
The calibur line is an excellent one, but I don't think the charge attack synergizes well with bombs. It essentially IS a bomb, except that you have to be standing where it goes off. Better off with a blast bomb that not only charges faster, but you can step away from.